The growing importance of wind energy in recent years in Spain, Europe and the rest of the world is well known, and forecasts point to sustained growth in the generation of wind energy worldwide. The energy policies of the most advanced and economically powerful countries include an increased presence of wind energy among their objectives.
Within this context, offshore wind farms are starting to appear, thus confirming the forecasts of sharp growth in the application of this technology in forthcoming years. While wind farms built on offshore sites are undoubtedly more expensive, logically depending on the depth of the waters where they are installed, the wind has greater quality, higher speed and less turbulence and, consequently, the number of production hours is higher which, added to the greater air density at water level, generates greater revenues than land-based farms, compensating the cost overrun of the initial investment.
The development and construction of offshore wind farms is frequent and the number of marine wind farms currently under study has grown significantly, particularly in Germany, the British Isles and Scandinavian countries, consistent with the predicted growth of these types of farms, closely linked to the strategic objectives established at state level aimed at reaching certain renewable energy quotas. The tendency to use higher-powered and larger wind turbines with the objective of reducing the unit cost of installed power has been ever-present in wind turbine development and is, if possible, even more accentuated in the case of offshore wind energy. Practically all large wind turbine manufacturers have high-power models, three-megawatt or more, under study or in advanced stage of development, adapted to sea conditions, which are particularly demanding. This, in turn, represents a significant increase in substructure-related specifications and requirements—foundation and shaft—imposed on the wind turbines which, added to their use in increasingly deep sites, will require the development of novel concepts for said substructure, with increased capacity and competitive cost.
The solutions generally envisaged in the current state of the art for the construction of offshore farms are listed and described below in an orientative and non-limiting manner.
Shallow water depths:                Driven metal monopile not connected to the tubular metal tower shaft itself.        Gravity-based foundations: structural concrete footing, often with pedestals. These are transported and anchored using barges and/or sea cranes.        Suction bucket: based on driving watertight buckets into the seabed and consequently leveraging the differences in pressure generated.        
Medium and deep water depths:                Tripod: The metal tower is supported by a structure having three tilted legs that rest on the seabed by means of driven piles or other similar system. The tower may be centered in relation to the tripod legs or disposed on one of said legs.        Tripile: The metal tower rests, by means of a cross-shaped transition part having three arms, on three vertical piles submerged and driven into the seabed.        Jacket: The metal tower is supported by a jacket structure having four legs or columns.        
In the case of ultra-deep water depths, floating solutions anchored to the seabed have been envisaged.
An overview of the state of the art results in the following general considerations:                All solutions are based on shafts in the case of metal tubular-type towers.        Solutions for medium and deep water depths include a change in tower shaft typology, with a metal tubular tower for the emerged part and a highly differentiated element for the submerged part (tripod, jacket, etc.).        Concrete gravity-based foundations are envisaged for shallow depths, such as semi-submerged structures, and include installation by means of sea cranes.        
Among the main drawbacks and limitations of the known solutions envisaged for the substructure of an offshore wind turbine, the following must be highlighted:                High costs deriving from the scarce and expensive means for transporting, handling and lifting the foundation, tower and turbine elements at sea.        Low durability of steel in marine environments due to the aggressive environmental conditions (high humidity/salinity), particularly in tidal zones, entailing high and expensive maintenance requirements. This, added to the high sensitivity of metal structures to fatigue loads, limits the useful life of the metal components of the substructure.        Highly sensitive to collisions with sea vessels, icebergs and drifting objects in general.        Highly dependent on complex and uncertain geotechnics in the different cases of gravity-based foundations.        In cases of ultra-deep water depths: complex, delicate and expensive transition zones between the emerged tubular shaft of the tower and the different types of partially submerged elements connected to the foundations at seabed level.        High environmental impact of driven pile solutions due to the noise and vibrations generated by these during execution thereof.        Uncertainties deriving from variability in steel pricing, notably more accentuated than that of concrete.        High sensitivity to critical connection details with foundations by means of driven piles, which must support the low redesign accuracy of driven solutions and have been a source of significant pathologies in current farms.        Metal tubular towers are based on factory-made, closed-circumference tube parts which limits maximum diameters if road transport is required. This limits tower capacity and height. If larger diameters than those transportable by road are sought by manufacturing the towers in shipyards or coastal facilities, this will considerably limit the potential industries and factories for manufacturing these towers.        Solutions involving limited tower shaft rigidity, which limits capacity for greater tower heights and wind generator sizes, particularly with low-rigidity foundation solutions, which is the most frequent case in offshore installations.        Expensive elements for the submerged part of the installation, increasing exponentially with depth.        High dependence on specific means for lifting and transport in marine environments, which are very limited in range.        